134 PLANT PHYSIOLOGY 
not reach their full size or number (hypoplasy), orin which 
individual cells or whole tissues are enlarged above the 
normal size (hypertrophy), or in which the cells are ab- 
normally increased in number (hyperplasy). In some 
eases cells destined to produce one kind of tissue are 
changed into other kinds by the pathological conditions. 
Furthermore, the internal structures of the cell may be 
modified. The chloroplasts may be increased in number 
and size or diminished or apparently wholly suppressed. 
The nucleus may be enlarged and changed in shape or 
caused to divide abnormally so that multinucleate cells 
result. The contents of the cells are often modified; 
acids may be increased or diminished; the tannin content 
may increase remarkably in some cases as also that of 
various coloring matters or of various enzymes. 
196. These changes are in some cases the results of 
causes not as yet recognizable. Such troubles are spoken 
of as “ Physiological Diseases,” this being simply a name 
to cloak our ignorance of the true cause of the trouble. 
In many cases, however, the changes occur as a result of 
the action of parasitic organisms, either plant or animal 
in nature. In the case of many injuries caused by animals 
(e.g. biting insects) the injury is chiefly mechanical and is 
a subject for study from the standpoint of pathology in 
just the same way as the study of wounds caused by other 
agencies. But the punctures of some insects (e.g. plant 
lice, aphids) are followed by marked physiological dis- 
turbances in the cells immediately or even remotely ad- 
jacent to the punctures, leading to the type of disease 
called stigmonose (or puncture disease). ‘Theenormously 
varied structures found in insect galls as a result of the 
presence or punctures of various gall-producing insects, 
if properly understood, would doubtless throw a flood of 
light upon the subject of pathology and even physiology. 
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